It's not necessarily a question of how many amps the motor is pulling. They could say that is the limit to prevent customers from running a bad setup, but typically KV limits are due to the firmware on the ESC's processor. The firmware controls the pulses being sent to the motor generally following a 6 step commutation pattern. The faster the motor can spin the faster the ESC has to turn on and off the MOSFETS in order to apply power to the correct phase to keep the motor spinning. Firmware efficiency, hardware optimization, and processor clock speed all play a role in how fast of a motor the ESC can run.

Castle car ESCs generally have a max Electrical RPM limit of about 120k. Electrical RPM is calculated by (Motor KV * Max Pack Voltage * Pole Count) / 2. So if you had a 3500KV 2 pole motor on 3s it would have an Electrical RPM of (3500*3*4.2*2)/2 = 44100. If that truly is their limit, then they may be running their processor at a lower clock speed.